"A very nice letter from Vladimir Putin; his thoughts are so correct," Trump said in a mid-day statement.

Trump's office released the letter, dated Dec. 15, in which Putin said he hopes to work with the new U.S. president in a "constructive and pragmatic manner" in order to "restore the framework of bilateral cooperation in different areas."

Said Trump "I hope both sides are able to live up to these thoughts, and we do not have to travel an alternate path."

U.S.-Russia relations will certainly be a major feature of the Trump presidency; the intelligence community has accused Putin's government of hacking e-mails from Democratic Party officials in an effort to help Trump win the recent presidential election. In 2014, Russia annexed the territory of Crimea from neighboring Ukraine, which triggered several rounds of economic sanctions against Russia by the United States and European Union.

Lawmakers in both parties are pushing an investigation into Russian involvement in the election.

Putin has denied the allegations; Trump, who made better relations with Russia a theme of his campaign, has questioned whether the Russians were involved in the hacking scheme.

Trump revealed the exchange with Putin on a day in which he criticized how conflict of interest laws affect his son, golfed with Tiger Woods, blew off reports that celebrities are shunning his inauguration, and re-affirmed his Thursday tweet about strengthening and expanding the U.S. nuclear ability -- another issue involving Russia and Putin.

Trump told MSNBC he is willing to engage in an arms race with other countries if necessary, though his aides said he is more interested in modernizing existing nuclear weapons systems.

"Let it be an arms race because we will outmatch them at every pass and outlast them all," Trump said, reported MSNBC's Morning Joe.

The comment came a day after Trump tweeted: "The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes."

In between tweets, Trump hit the links Friday with Woods at Trump International Golf Club in West Palm Beach, Fla. Trump is spending the holiday weekend at his Mar-a-Lago estate in Palm Beach.

Various nuclear comments Friday reflect the difference between Trump has said in person and via Twitter and what his staff tries to interpret after Trump speaks.

Trump spokesman Jason Miller said Thursday afternoon, hours after Trump's initial tweet, that the president-elect referred to "threat of nuclear proliferation and the critical need to prevent it — particularly to and among terrorist organizations and unstable and rogue regimes. He has also emphasized the need to improve and modernize our deterrent capability as a vital way to pursue peace through strength.”

Incoming White House press secretary Sean Spicer, also Friday on MSNBC's Morning Joe, said Trump is issuing a warning to other countries. They "need to understand that ... if they expand their nuclear capabilities this president is not going to sit back, he's going to act," Spicer said.

Russian President Vladimir Putin, who has ordered his military to strengthen its "nuclear potential," told reporters that his existing system is "more efficient" that the U.S. one, and can pierce any missile defense system.

Putin also said he is not interested in engaging in an arms race, and that he found nothing unusual in Trump's comments.

"We will never look to be dragged into an armed race and to spend resources that we can't afford," Putin said in his end-of-the-year news conference, and that he understands the United States has the stronger military.

Putin also praised Trump, saying: "He went to the end, though nobody believed that he would win except us."

Meanwhile, also on social media, Trump blamed conflict-of-interest laws for the decision by his son Eric to sever involvement with a charity.

In a pair of tweets, Trump said: "My wonderful son, Eric, will no longer be allowed to raise money for children with cancer because of a possible conflict of interest with my presidency. Isn't this a ridiculous shame? He loves these kids, has raised millions of dollars for them, and now must stop. Wrong answer!"

Ethics lawyers have raised concerns that Trump and his family could be perceived as raising money off the presidency.

Trump also appeared to react to reports that several entertainers have refused to perform at inauguration events next month.

Tweeted Trump: "The so-called "A" list celebrities are all wanting tixs to the inauguration, but look what they did for Hillary, NOTHING. I want the PEOPLE!"